1. Field of the Invention
The invention relates to memory associated with the central processing units of microcomputers and more particularly, in such memory, to an architecture and device used to protect certain zones of the memory against unauthorized read operations.
2. Discussion of the Related Art
Microcomputers are used in many applications such as chip-based bank cards and mobile telephone sets. The possibilities offered to users may vary from one user to another depending on the characteristics of the subscription, and the price of a subscription depends on the possibilities to which it gives access.
To minimize the cost of the microcomputers, the microcomputers, also called microcontrollers, used in such applications are designed to fulfill all the functions available, however access to these functions is limited depending on the type of subscription. The limiting of access to certain functions is done during the manufacturing process through the prohibition of access to certain zones of the memory that correspond to functions not paid for in the subscription.
The zones of the memory that correspond to functions of the subscription that has been paid for may be identified by the codes C1, C2, C3, . . . , CN so that, broadly speaking, a code C1 corresponds to the enjoyment of basic functions while the code CN corresponds to the enjoyment of all the functions available.
It is important to prevent fraudulent identification of these codes C1 to CN. Such identification would allow access to all the functions available with CN, while requiring payment only for the subscription that corresponds to the basic functions, i.e., C1.